This disclosure relates generally to a cholesteric eWriter, for example, utilizing a pressure sensitive writing tablet as referred to in the prior art. In general, Bistable Liquid Crystal Displays, and in particular, Cholesteric Liquid Crystal Displays (ChLCDs), (also called chiral nematic liquid crystals) have proven to have great potential to create low cost pressure sensitive eWriters that are efficient power consumers and that can be utilized in a number of unique devices.
Recently, the BOOGIE BOARD® pressure sensitive cholesteric liquid crystal eWriter of Kent Displays Inc. has appeared on the market in which a pointed stylus or the finger can be used to write or trace an image on the surface of the tablet as described in U.S. Pat. No. 6,104,448, which is incorporated herein by reference in its entirety. As used in this disclosure, the term eWriter includes any device including substrates and an electrooptical material disposed between the substrates which exhibits a change in reflectance as a result of writing pressure applied to one of the substrates, the device being able to be erased upon application of a voltage. This definition of an eWriter excludes devices that are only able to be written on by digitally electrically addressing the liquid crystal as in the case of a laptop that employs a display actively driven with a TFT array. The cholesteric liquid crystal eWriter offers a considerable improvement over previous technologies in that the image can be simply and instantly erased with the push of a button that applies a voltage pulse to electrically conductive electrodes in the eWriter. In a cholesteric liquid crystal eWriter, the liquid crystal, dispersed in a polymer network, is sandwiched between two substrates that are spaced to a particular gap. Cell gap is used interchangeably with gap in this disclosure and is the distance between electrodes. The cholesteric liquid crystal eWriter polymer network can be created by several polymer dispersed liquid crystal types; including Polymerization Induced Phase Separation (PIPS), Thermally Induced Phase Separation (TIPS), or Solvent Induced Phase Separation (SIPS), as referred to in Drzaic, P. S. (1995) Liquid Crystal Dispersions. Singapore; World Scientific, which is incorporated herein by reference, and are well known to one skilled in the art of eWriters. PIPS is the use of light to initiate polymerization and ultimately cause the liquid crystal molecules and polymer structure to phase separate. TIPS is the use of temperature to initiate the chemical reaction thus resulting in the liquid crystal phase separating from the polymer structure. SIPS is the use of solvent evaporation to cause the liquid crystal to phase separate from the polymer structure.
Both substrates of the cholesteric liquid crystal eWriter are flexible and the bottom substrate is painted with an opaque light absorbing dark background. The upper substrate and upper electrode are optically clear. Within the gap is a bistable cholesteric liquid crystal which can exhibit two textures, a substantially transparent (focal conic) texture and a color reflective (planar) texture. The spacing of the cell gap is usually set by plastic or glass spacers that are either cylindrical or spherical in shape. The cholesteric liquid crystal eWriter is initialized by applying voltage pulses to the electrodes to electrically drive the cholesteric material to the substantially transparent texture. When one presses on the top substrate with a pointed stylus or finger, the liquid crystal is locally displaced. Flow induced in the liquid crystal changes its optical texture from substantially transparent to a reflective color at the location of the stylus. The reflective color contrasts well with the dark background of the lower substrate. An image traced by the stylus or finger nail will remain on the tablet indefinitely without application of a voltage until erased. Erasure is accomplished by applying a voltage pulse to transparent conducting electrodes on the inner surface of the two substrates that drive the cholesteric liquid crystal from its color reflective state back to its substantially transparent state.
The above described principle is disclosed in more detail in U.S. Pat. No. 6,104,448. Dispersions of liquid crystal in a polymer matrix can be used to control the pressure sensitivity and resolution of the image as described in U.S. Pat. No. 8,228,301, which are suitable for use in the cholesteric enhanced brightness eWriter of this disclosure, this patent being incorporated herein by reference in its entirety. Other modes of operation and a cholesteric liquid crystal eWriter that can produce multiple color images are described in this U.S. Pat. No. 8,228,301 patent, and a means for select erase is disclosed in U.S. Pat. No. 8,139,039, incorporated herein by reference in its entirety, all of which are suitable for use in the cholesteric eWriters of this disclosure. One mode of operation, known as Mode A, provides a negative image from that described above. In that mode the tablet is initialized by electrically driving the tablet display to the color reflective planar texture with a voltage pulse or pulses. Then one can write images by driving the cholesteric material to the substantially transparent texture with the pressure of a pointed stylus and simultaneous application of a voltage to the electrodes sandwiching the cholesteric liquid crystal. This mode of operation with a color reflective background is termed Mode A whereas the other mode with a transparent background is termed Mode B.
The commercial BOOGIE BOARD® cholesteric liquid crystal eWriter, operated in Mode B, has the color black for the fixed opaque light absorbing background. The dark black background offers high contrast for the color reflective image written on the tablet. As disclosed in U.S. Pat. No. 5,493,430, incorporated herein by reference in its entirety, other opaque colors may also be used for the fixed background of a cholesteric liquid crystal display. The color of the background additively mixes with the reflective color to present a different color than that of the cholesteric liquid crystal. There may be multiple colors on the background and those colors may be patterned. As an example, the pattern could be lines offering a lined tablet for convenience in writing text similar to a ruled paper tablet.
A problem with the prior art is that the reflectivity is low for a written texture and needs to be improved. There is always a desire for higher brightness devices to write and draw on. The prior art discloses cholesteric liquid crystal eWriters with written reflectivity and contrast ratio that is sufficient for seeing the handwritten work, but for a more desirable writing experience improvement is needed. An understanding of the prior art is that to increase reflectivity from a typical cholesteric display that is electrically driven to the planar texture, then the cell gap of the display must be increased [W. D. St. John et. al, “Bragg reflection from cholesteric liquid crystals,” The American Physical Society, 1191-1198 (1995)].